In an active matrix apparatus that includes a pixel array unit in which pixels including electrooptic elements are arranged in a matrix with a large number of rows and columns and a scan line is wired for each row and a signal line is wired for each column with respect to the pixel arrangement, a vertical driving circuit configured to select each pixel in the pixel array unit on a row-by-row basis, and a horizontal driving circuit configured to write a video signal into each pixel in a row selected by the vertical driving circuit, a dot sequential driving mode is a method that sequentially samples, e.g., analog video signals that are serially input over one horizontal scanning period and writes the sampled video signals into a corresponding signal line in the pixel array unit.
For an active matrix display apparatus that uses the dot sequential driving mode, when the number of pixels in a horizontal direction, in particular, is increased as the display apparatus realize a higher resolution, it becomes difficult to maintain a sufficient sampling period of time for sequentially sampling video signals input by one system with respect to all pixels within a limited horizontal effective period. Therefore, in order to maintain a sufficient sampling period of time, an m-dot simultaneous sampling driving mode, in which video signals are input in parallel in m systems (where m is an integer equal to or greater than two) and, with m pixels (dots) in the horizontal direction used as a unit, m sampling switches are provided and simultaneously driven by one sampling pulse to sequentially perform writing in units of m pixels, has been used (see, for example, Japanese Unexamined Patent Application Publication No. 2003-066914, in particular, paragraph 11 and FIG. 16).
As image quality and resolution in image display apparatuses increase, in projection liquid crystal display apparatuses (LCD projection apparatuses), for example, there is increasingly a demand for quantum extended graphics array (QXGA), which is a graphics display standard that supports approximately 3 million pixels (2048(H) by 1536(V) pixels).
A projection LCD apparatus is a display that uses a liquid crystal panel (liquid-crystal light valve) as an optical switching element and projects on a screen an enlarged image of an image on the liquid-crystal light valve by using a projection optical system.
In such a projection LCD apparatus, an active matrix LCD apparatus used as a liquid-crystal light valve uses a 12-dot simultaneous sampling driving mode (m=12) in a case where extended graphics array (XGA), which is a currently used graphics display standard that supports 1024(H) by 768(V), is used. In a case where the QXGA display standard is used, since the number of pixels in QXGA is four times larger than that in XGA, the simultaneous sampling number, m, is inevitably increased. Typically, the number of simultaneous sampling dots in QXGA is set to be four times larger than that in XGA, as is the case with the number of pixels, and therefore, a 48-dot simultaneous sampling driving mode is used.
However, if the simultaneous sampling number, m, is increased, a problem to be solved arises in which the waveform of a sampling pulse for driving a horizontal switch for sampling a video signal and writing it into a signal line is more rounded, which is caused by resistance and capacitive load in transients. Such delay in sampling pulses and waveform rounding thereof is a factor that causes ghosts. A cause of ghosts is now described below. The cause of ghosts occurring when the peak of black level contained in a video signal is written into a pixel column in an Nth stage (Nth column) is schematically illustrated in FIG. 10.
At an early point, i.e., before aging, which is performed to stabilize operation by the passage of electric current through the apparatus, no delay in the sampling pulses occurs, so that the black level of a video signal can be accurately sampled with the sampling pulse in the Nth stage. Therefore, no front ghost is present. In contrast to this, after aging, delay in the sampling pulses occurs, so that the peak of black level may be sampled in part with a drive pulse of the preceding stage ((N−1)th stage) in some cases. If so, front ghosts are present.
Specifically, if a liquid crystal panel is used for a long time, a threshold voltage Vth is increased because of the presence of hot carrier stress of transistors disposed in a circuit system through which sampling pulses pass. As a result, the sampling pulses are displaced in a rear direction in time and front ghosts are thus present. In particular, when thin film transistors (TFTs) are used as the transistors, the width of delay in the sampling pulses caused by hot carrier stress is on the order of 30 nanoseconds.
For an active matrix LCD apparatus, in a case where 1H inversion driving method, which inverts the polarity of a video signal to be written into each pixel for every 1H (H is a horizontal scanning period), is used, a video signal on a signal line jumps into a common line or scan line because of coupling of parasitic capacitance between the signal line and the common line or that between the signal line and the scan line. This increases the amount of variation in the potential of the common line and the scan line. Therefore, as shown in FIG. 11, horizontal crosstalk (A) and a window band (B) are clearly present, and as a result, image quality is severely degraded. FIG. 11 illustrates phenomena caused by an event in which variation in the potential of the signal line jumps into the common line or the scan line through coupling when the black level is written into the signal line.
The present invention aims to solve the problems described above. An object of the present invention is to provide a display apparatus and method for driving the display apparatus that are capable of suppressing image quality degradation resulting from delay in transmission of sampling pulses or from waveform rounding thereof and image quality degradation caused by coupling between the signal line and the common line and that between the signal line and the scan line even when the simultaneous sampling number m is increased.